A conventional commercial aircraft generally includes a fuselage, a pair of wings, and a propulsion system that provides thrust. The propulsion system typically includes at least two aircraft engines, such as turbofan jet engines. Each turbofan jet engine is typically mounted to a respective one of the wings of the aircraft, such as in a suspended position beneath the wing, separated from the wing and fuselage.
Initially, the turbofan jet engines must be started. Additionally, it may be necessary to start, or more particularly to re-start, one or more of the turbofan jet engines during a flight in the event of an unscheduled shutdown of the turbofan jet engine. For example, the turbofan jet engine may shut down due to ice ingestions, bird strikes, inadvertent engine shutdown commanded by the flight crew, etc.
During an initial start, a starter system, such as a pneumatic starter, rotates the engine to a speed sufficient to initiate an ignition of a combustion section of the turbofan jet engine. With regard to mid-flight starts, there are currently at least two types. The first type of mid-flight engine start is an “assisted” start in which the engine is rotated by both an airflow through the engine and torque exerted by the starter system. The second type of mid-flight engine start is typically referred to as an “unassisted” start because the normal engine starter is not utilized to accelerate the engine. Instead, only ambient airflow through the engine generates torque that causes the engine to rotate, prior to lightoff, to a minimum speed. This torque associated with airflow through the engine is typically referred to as “windmilling” torque. The torque and resultant minimum pre-lightoff speed in the unassisted case are typically low relative to those obtained in an assisted start operation.
Assisted starts are typically preferred to unassisted starts because the additional starter torque allows the engine to achieve idle faster, more reliably and with less overall stress on the engine due to high exhaust gas temperatures, etc. Regardless, engines are required to start in an unassisted manner under certain conditions due to the unavailability of power to the starter system. In most cases the starter system is powered by pneumatic energy provided by either the other aircraft engine or by the Auxiliary Power Unit (APU). If neither is running or running efficiently, pneumatic power might not be available to the starter system and hence a restart must be of the unassisted type.
When performing an unassisted start, engine lightoff must occur at a minimum speed that is determined by the windmilling torque. This minimum speed is low relative to the assisted case, and accordingly the main fuel pump (which is driven through rotation of the core) is typically oversized to be able to provide sufficient fuel to start the engine at this minimum speed. Additionally, the lack of starter system torque in the unassisted case leads to higher internal temperatures, slower acceleration rates, and reduced compressor stall margin at any given acceleration rate when compared to an assisted start. The factors may reduce a likelihood of a successful start, increase a time required to perform the restart, and potentially decrease a lifespan of certain components within the engine due to elevated internal temperatures.
Similarly, during assisted starts, traditional starter systems often provide sub-optimal torque contribution due to the limited torque/power capacity of the starter system and lack of ability to operate the starter at speeds approaching or surpassing engine idle speed. This sub-optimal torque contribution is worsened during high altitude and in-flight starts, as the systems providing pneumatic power to the starter system normally experience a reduction in capacity when ambient pressure is reduced (i.e. when operating at higher altitudes).
Accordingly, a propulsion system having features providing for higher probability starting of a gas turbine engine, more efficient starting of a gas turbine engine, and/or quicker starting of a gas turbine engine would be useful.